Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Do I need the filter with a filter?

  • Please log in to reply
7 replies to this topic

#1 Mark Bailey

Mark Bailey

    Ranger 4

  • -----
  • topic starter
  • Posts: 337
  • Joined: 05 Jan 2009
  • Loc: Torrey, Utah

Posted 02 December 2019 - 10:44 PM

I am contemplating putting my Gary Honis full mod Canon Rebel 500D on a refractor with a filter wheel. I think all the filters are in the wheel are IR blocked. Will I need the Astronomik "L" Luminance UV/IR filter clipped in? I am thinking not, but would appreciate some advice.

 

Thanks as always,

 

Mark


  • Thaeland likes this

#2 scadvice

scadvice

    Surveyor 1

  • *****
  • Vendors
  • Posts: 1,558
  • Joined: 20 Feb 2018
  • Loc: Lodi, California

Posted 02 December 2019 - 11:27 PM

I'm not sure you can do that because there is not enough backfocus distance (55mm?). If you can I'd sure like to know.


  • Mark Bailey likes this

#3 Samir Kharusi

Samir Kharusi

    Mercury-Atlas

  • *****
  • Posts: 2,605
  • Joined: 14 Jun 2005
  • Loc: Oman

Posted 03 December 2019 - 03:07 AM

Just VERIFY that all the filters in your filter wheel are blocking IR, via the filter maker's website. If so, no additional filter is required.


  • Mark Bailey likes this

#4 Mark Bailey

Mark Bailey

    Ranger 4

  • -----
  • topic starter
  • Posts: 337
  • Joined: 05 Jan 2009
  • Loc: Torrey, Utah

Posted 03 December 2019 - 11:14 AM

Thank you, Samir. These filters were purchased by my now elderly father. A little sleuthing and I see that the LRGB is an IR set from Optec. The H-alpha filter I am not finding records for. So let me ask a couple more questons:

 

  1. Does a narrow band filter like a Ha filter block IR by feature of it being narrow band? 
  2. Is it better/okay to be safe than sorry and leave the Luminance UV/IR filter clipped in to use in conjunction with the Ha filter in the filter wheel?
  3. What happens if I leave the Luminance UV/IR filter clipped in when I use it with the LRGB-IR filter wheel filters?

-Mark


Edited by Mark Bailey, 03 December 2019 - 12:16 PM.


#5 Samir Kharusi

Samir Kharusi

    Mercury-Atlas

  • *****
  • Posts: 2,605
  • Joined: 14 Jun 2005
  • Loc: Oman

Posted 04 December 2019 - 03:55 AM

3. Just introduces an extra couple of sources of bright-star flares. That piece of glass has 2 sides... Otherwise no major harm.

2. for the above reason, leave out

1. Presumably your Dad used the filter wheel with a mono astroCCD. Such CCDs normally have a sensitivity similar to a full-spectrum mod on a DSLR. Consequently I would expect that filters meant for use with a mono astroCCD will have UV/IR blocking. No need for your Luminous filter.

 

Try this simple test to learn a bit more how filters and modded DSLRs behave. Point you OTA at a blank wall during daytime. Set your DSLR to auto shutter speed (your OTA has a fixed aperture). Note down the shutter speed, let us assume it says 1/1000 second. Remove the Luminous filter and note down the new auto-shutter speed. I would expect it to be faster, perhaps around 1/2000 second. This is because a full spectrum mod has half of its sensitivity in the IR. Now insert the NB Ha filter (with the Luminous filter out). Your auto-shutter speed will now be slower because you have cut off most of the available wavelengths, visible and IR. I would expect the new auto-shutter speed to be roughly 3x slower, say, at 1/250 second. Now insert the Luminous filter in addition. Three possible outcomes: A. the new auto-shutter speed is only marginally slower, say, 1/200 second, indicating that your Luminous filter still passes Ha but does block IR. B. The auto-shutter speed is much slower, say, 1/50 second, indicating that your Luminous filter blocks Ha (i.e. a true visible-light only filter). or C. The auto-shutter speed is much slower because your Ha filter was passing IR.

 

I'll have to think of another test to distinguish between B and C... I expect you will get A. If not, just post all your shutter speeds and we'll go from there.


  • Mark Bailey likes this

#6 Mark Bailey

Mark Bailey

    Ranger 4

  • -----
  • topic starter
  • Posts: 337
  • Joined: 05 Jan 2009
  • Loc: Torrey, Utah

Posted 04 December 2019 - 11:06 AM

Samir, this is terrific help. Thank you very much! At the moment I am at a remote location from the observatory and Dad's SBIG 10 XME CCD camera is mounted with the filter wheel. Your advice that filters designed for a CCD camera would have UV/IR blocking is very helpful and seems like a safe assumption. The filter wheel has 5 stations all taken with LRGB and Ha. Dad has other blank wheels that with a little fiddling (learning how to program a new wheel) I could use to accomplish your tests including no filters. It will be at least a couple of weeks so please stay tuned.

 

One theoretical question - if a Ha filter is a narrow band that is not in IR wavelengths(?), would IR not be automatically blocked? Your answer about my question 1 above suggests perhaps not.

 

-Mark

 

PS- great website! Definitely not boring. I noticed a recent thread here asking about what to do during a long, boring night. The topic starter should check in with you.


Edited by Mark Bailey, 04 December 2019 - 11:24 AM.


#7 Samir Kharusi

Samir Kharusi

    Mercury-Atlas

  • *****
  • Posts: 2,605
  • Joined: 14 Jun 2005
  • Loc: Oman

Posted 05 December 2019 - 01:32 AM

 

One theoretical question - if a Ha filter is a narrow band that is not in IR wavelengths(?), would IR not be automatically blocked? Your answer about my question 1 above suggests perhaps not.

 

-Mark

 

PS- great website! Definitely not boring. I noticed a recent thread here asking about what to do during a long, boring night. The topic starter should check in with you.

This is what a full spectrum mod DSLR sees:

large.jpg

 

Ha is at 6563 Angstroms. Astro filters are fabricated by depositing various layers of varying thicknesses with different refractive indices so that the desired wavelengths end up with either constructive interference or destructive. A simpler design, e.g. a UHC filter for visual use, may allow IR longer than 7000 A to pass through freely. Your full spectrum mod would, however, be quite sensitive between 7000 and 9500. Such a filter will require a supplemental UV/IR blocker. Hence, e.g. Astronomik offers a UHC-CCD filter that has IR blocking built in. A NB Ha filter is very unlikely to have been designed for visual use, lousy sensitivity of human vision to Ha, but one never knows for sure... Here's waht happens with an Astronomik UHC filter originally designed for visual use:

uhc.jpg

 

And adding a UV/IR blocker:

 

uhc-uibar-lines.jpg


  • Mark Bailey likes this

#8 Mark Bailey

Mark Bailey

    Ranger 4

  • -----
  • topic starter
  • Posts: 337
  • Joined: 05 Jan 2009
  • Loc: Torrey, Utah

Posted 05 December 2019 - 01:56 PM

Thanks yet again. Samir. This is great to learn.  I think I am getting the idea. -Mark


Edited by Mark Bailey, 05 December 2019 - 02:25 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics